Question on rectifier tubes

[donald stringer]

   A light just came on, rectifier tubes in tubes circuits perform the same function as solid state rectifiers do... they change a/c to dc. Am I correct ?

[Papa_lazerous]

A rectifier tube performs the same function as a  silicon/germanium rectifier.... Where do you think the name Diode came from? thats the true name of a rectifier tube. then they put a control grid in there and you have a triode.....uh huh the penny drops not so many people realise that even though it is a little OT

[donald stringer]

I"ve looked at those things as I"ve been studying tubes and such and its funny I didnt see that. By the way but what does " a penny drops" mean

[brett]

Hi
in some respects valves don't do the job as well as solid state devices (low current, higher impedance).  However, the high impedance (usually a few dozen ohms) causes the classic "sag" when running a tube-rectified amp flat out.
cheers

[sean k]

Are you correct that rectifier tubes change AC to DC... nope they don't. They help though by being a diode that cuts the negative going wave and leaves only the positive part of the wave. It's still AC though but instead of being from negative 200V peak to 200V peak it's now from 0 to 200V peak. The chokes and/or capacitors when then put from rail to rail become the makers of DC by storing the alternating voltage.

[JKowalski]

You typically see them used with the two anodes connected to each end of a center-tapped trasformer, with the center tap to ground. This works pretty much the same as your classic center tapped dual diode power supply - you will get a full wave rectified AC signal on the cathode. You still have to filter it to get DC.

How the tubes work is simple. The cathode is heated - this makes the electrons in it able to overcome the forces holding them there easily, and escape into the vacuum. When you put another plate by it (anode), and make it more positive in voltage then the heated cathode, the loose electrons are pulled to the anodes. Since the anodes are not heated, the electrons are retained in them. Therefore, the electrons can only go from cathode to anode.

Remember - electrons go from negative to positive, even though we design circuits with the idea that current goes from positive to negative.

[BDuguay]

I have a general understanding of the Rectifier tubes function. When the generous and helpful forum members pipe in and start talking about 'anodes' and 'cathodes' and 'control grids' I can't help but feel incompetant(?).
Sadly, I too am more interested in what 'the penny drops' means.
B.

[MikeH]

In amplifiers over 50W or so most amp designs switch to solid state diode rectification, apparently because sag can get pretty bad.  A good example would be the super reverb and twin reverb.  Aside of the speaker difference the super is almost an exact 1/2 power version of the twin, except it uses tube recto while the twin uses diodes.

[Papa_lazerous]

Well everything has been covered but "the penny drops" I will ellaborate on, Its and English saying that hasn't made its way over to your side of the Atlantic.

I will try and explain in context....

Say for example you are explaining something to someone, and the other person just doesn't get it.  Then as you explain some more they are all "Ahhhhh now I get it" thats the penny dropping

Might not make any sense but thats it.

[donald stringer]

Ah Ha  The penny just dropped again !  And along with what every one says above tube rectifiers arent fully rectifying the ac ..... only half wave rectification and the only thing that may make them superior is unless you want the sag.

[JKowalski]

Ah Ha  The penny just dropped again !  And along with what every one says above tube rectifiers arent fully rectifying the ac ..... only half wave rectification and the only thing that may make them superior is unless you want the sag.

Well, no - like I said - they are most commonly in a center tapped transformer layout, and the most common rectifier tube is basically two anodes and one cathode - so, in solid state terms, two diodes with their cathode (the line side) connected together.

See here - this is the solid state version of the power supply i am talking about:



The one rectifier tube (with the two anodes and the cathode) can sub in for the two diodes. It will work exactly the same - you get a full wave rectification.

Here is the tube version of the above:



As you can see, it's exactly the same -there are some things which may make it look different, but I will explain them. The cathode (triangle thing) in the tube has two wires coming out of it. The top one is the output - equivalent to the point where the two solid state diodes are connected together. The wire coming from the bottom is just the heater - it heats the cathode with a lower voltage. The reason they attached it directly to the cathode is just because it's easier to have 4 pins instead of five on the tube, and it heats the cathode more efficiently (because the heating filament is the cathode). The two capacitors and the inductor are just filtering - a more advanced version of the solid state examples one capacitor. It changes the full wave rectified signal to DC. The bottom right dot is ground, the top right dot will be your DC power.

Advantages of tube rectifiers:
- If you use tubes in the rest of your device, the slow warming up of the power supply tube prevents a sudden current inrush to the other tubes before they too turn on. Basically, it slowly gives power to the tubes at the same rate they start to function. It's easier on your tubes.
- They have sag, if you want that
- They can work well with ridiculously high voltages that some solid state devices cant cope with (I don't think you'll need to consider that)
- Internal resistance

Advantages of Solid State Rectifiers
- Smaller
- Cheaper
- Don't need heating
- Behave more like the "ideal" diode (infinite current capability in one direction, zero in the other)

[donald stringer]

Thats very clear now thanks. I have a jar full of those tubes someone gave to me.